Solid oxide fuel cells (SOFCs) are efficient energy conversion devices with flexible selection of hydrocarbon fuels. Currently, downscaling of electrolyte thickness to reduce ohmic resistance has been an effective way for further improving the performance of SOFCs at low operating temperatures below 500° C. For drastic reduction of electrolyte thickness to nanometer-scale, silicon-based microfabrication process using chemical etching has been successfully utilized for SOFCs.
Thin film electrolytes with nanoscale thicknesses of between 50 to 150 nm were fabricated previously by MEMS-based microfabrication processes with atomic layer deposition (ALD), sputtering, or pulsed laser deposition (PLD). Further, fuel cell performances from the yttria-stabilized zirconia (YSZ) electrolytes with superior power densities above 1000 mW/cm2 at 500° C. were also reported.
Nevertheless, successful low-temperature SOFCs with a nanoscale thin electrolyte are currently only feasible in miniature scale due to severe residual stress on the membrane. A 50 nm thick, free-standing YSZ membrane is typically confined within only a few hundred micrometers in lateral dimensions, which limits the available electrochemically active area. Although superior power densities at reduced temperature have been reported elsewhere, the tininess of such SOFCs leads to insignificant power output of only at micro-watts scale, thereby limiting their applications as practical power sources.
It is virtually impractical to simply enlarge the size of such a thin membrane to increase surface area. Therefore, an effective method to scale up the nano thin film YSZ membrane SOFCs with robust mechanical strength of membranes is prerequisite for higher total power output.
To maximize electrochemically active area within a confined dimension, free-standing array μ-SOFCs has been fabricated. By creating free-standing corrugated YSZ electrolyte films from the pre-patterned silicon substrate, surface utilization was significantly increased by 30% to 64% on the silicon wafer. The array, which is 600 μm×600 μm, delivered a higher total power output of 3.1 mW at temperatures below 500° C. However, as individual cells featured free-standing and cup-shaped structure, there are many geometrical discontinuities, which may be mechanically weak points. The cells, which are arrayed on a square template, may undergo non-uniform membrane stress distribution. In particular, cells in the vicinity of each corner, where stress concentration experience is the highest, may be damaged, leading to membrane failures during fuel cell operation.
Kerman et al. (K. Kerman, T. Tallinen, S. Ramanathan and L. Mahadevan, Journal of Power Sources, 2013, 222, 359-366) reported stress behavior around the boundaries of square thin film SOFCs by numerical simulation and confirmed that stress was highly concentrated at the corners of the square membrane.
Su et al. (P. C. Su and F. B. Prinz, Electrochemistry Communications, 2012, 16, 77-79) presented electrolyte membrane array μ-SOFCs with silicon supporting layer to improve mechanical stability of the array membrane. Individual cells were supported by surrounding single crystal silicon and 6 mm by 6 mm square YSZ membrane electrolyte array was successfully demonstrated. Nevertheless, this structure has still stress concentration points at each corner of square templates.